When chitosan is prepared from the chitin fraction of the shells of crustacea, such as shrimp, crab and lobster, the final step is invariably and of necessity a treatment with extremely strong sodium hydroxide. This is the predominant industrial treatment that effectively removes acetyl groups from chitin, converting it into chitosan. Because of this treatment, chitosan is always found as an aminopolysaccharide with essentially none of the amino groups being protonated. This can be referred to as the free base form of chitosan. This natural form of chitosan is not soluble in water. In order to dissolve chitosan in water-based systems, the chitosan must be made more hydrophilic. This is done by adding acid to the water being used in dissolution. The acid reacts with the amino groups, converting them into (substituted) ammonium ions, which are much more hydrophilic than the amino groups. When chitosan is added to this acid-water mix, it becomes protonated. A fully protonated (i.e., each amino group has reacted with a proton from the acid) chitosan is the other extreme of chitosan. It should be noted, however, that essentially no protonation of chitosan will result if a dry acid and chitosan are merely physically blended. It is necessary to give the acid molecule mobility, which is done by the addition of water.
Chitosan is presently used as a dietary supplement to prevent some of the ingested fat in a person's diet from being absorbed and metabolized. It is thus an agent to help control obesity. When a person takes a dosage of chitosan, the chitosan exerts a demand on the stomach to produce hydrochloric acid in order to dissolve it. It is understood in the art that the chitosan must dissolve to be able to occlude the fat, which can thereafter be passed through the digestive tract and subsequently expelled from the body. Since the body's capacity for producing hydrochloric acid is limited, an agent that supplies part of the necessary acidity would be beneficial to chitosan's performance.
Merely treating chitosan with hydrochloric acid in a manufacturing process to form a water-soluble chitosan, however has an unintended and devastating side effect. It has been observed that these hydrochloric acid salts of chitosan undergo depolymerization upon storage, producing a product with too low a molecular weight to perform in the desired manner.
Presently, in the art, water-soluble chitosans are prepared by making a slurry of the chitosan in water and then adding acid to the slurry. Alternatively, one may make a solution of the acid and water and then add the chitosan under effective agitation conditions. It would be convenient, however, and represent an advance in the art, were all or part of the acid to be compounded with the chitosan thus providing a uniform dry complex which can be a shelf-stable product that will result in a reduced demand on the stomach for acid. To date, the only way to prepare such a complex has been achieved, with great difficulty, has been for the manufacturer to 1) dissolve the chitosan in aqueous acid, 2) filter the very viscous solution to remove insolubles, if necessary, and 3) spray dry the resulting solution to form a chitosan salt that is water soluble. Such a process is cumbersome, expensive and ineffective for an economically viable commercial process. Therefore, there is a need in the art for a more efficient and effective method of preparing chitosan salts.